Because of their high energy density, lithium ion secondary batteries are increasingly used in recent years as portable rechargeable power sources for laptop computers, mobile phones, digital cameras and digital video cameras. Also great efforts are devoted to the development of lithium ion secondary batteries as auxiliary power sources for electric automobiles and hybrid automobiles which are desired to reach a practically acceptable level as environment-friendly automobiles that release little or no exhaust gases to the air, or electric double-layer capacitors.
The lithium ion secondary batteries, albeit their high performance, are not satisfactory with respect to discharge characteristics in a rigorous environment, especially low-temperature environment, and discharge characteristics at high output levels requiring a large quantity of electricity within a short duration of time. On the other hand, electric double-layer capacitors suffer from problems including insufficient withstand voltage and a decline with time of electric capacity. Most such devices use non-aqueous electrolytic solutions comprising low flashpoint solvents such as dimethyl carbonate and diethyl carbonate as a main component. If thermal runaway occurs within the battery, the electrolytic solution can be evaporated and decomposed, inviting the risk of failure or ignition of the battery. This necessitates to incorporate an IC circuit in the battery as means for interrupting the current flow during anomalies and a safety valve for avoiding a rise of internal pressure within the battery due to generation of hydrocarbon gas. Further investigation on the electrolytic solution is pursued for improved safety, weight reduction and cost reduction.
Under such a situation, studies were made on polyether-modified siloxanes having high chemical stability and high compatibility with electrolytic solution. The polyether-modified siloxanes have an ability to dissolve electrolytes such as LiPF6 thoroughly and due to their own surface activation capability, help wet the electrode or separator. It is also known that the addition of only several percents of polyether-modified siloxane improves charge/discharge cycle performance. These effects, however, are not sufficient. The polyether-modified siloxanes are low in thermal stability and their use at low temperature is problematic because of relatively high melting point. There is a need for additives having high safety and high compatibility with electrolytic solution.
Reference should be made to JP-A 11-214032 and JP-A 2000-58123 both corresponding to U.S. Pat. No. 6,124,062, JP-A 2001-110455, and JP-A 2003-142157.